Attention has been given to lithium ion secondary batteries as a power source for driving electronic equipment. For example, graphite materials have an average potential of about 0.2 V (vs. Li/Li+) during desorption of lithium and, therefore, high-voltage lithium ion secondary batteries can be obtained by using graphite materials as a negative electrode active material. Further, graphite materials have a comparatively flat potential characteristic with respect to time during desorption of lithium. For these reasons, a lithium ion secondary battery containing a graphite material as a negative electrode active material is favorably used as a power source for a device which needs to have a high voltage and a flat voltage characteristic. Graphite materials, however, have a small capacity per unit mass of 372 mAh/g, and a further increase in capacity cannot be expected.
Silicon (Si), tin (Sn), oxides of Si and Sn and other materials capable of forming an intermetallic compound with lithium are considered promising as negative electrode materials which provide a higher capacity in comparison with graphite materials. However, the crystal structure of each of such materials is changed when the material absorbs lithium, resulting in a change in volume of the material. For example, in the case of Si, Si and Li form Li4.4Si when the amount of lithium absorbed in Si is maximized. The rate of increase in volume of Si with the change from Si to Li4.4Si is 4.12 times. In the case of absorption of lithium in graphite, on the other hand, the rate of increase in volume of graphite is 1.2 times even when the amount of absorption of lithium in graphite is maximized.
A large change in volume of an active material in the form of particles causes cracking of the active, material particles, imperfect contact between the active material and a current collector, etc., resulting in a reduction in charge/discharge cycle life. Particularly when cracking of active material particles occurs, the surface area of the active material particles increases and the reaction between the active material particles and a non-aqueous electrolyte is accelerated. As a result, a film derived from a component of the electrolyte is formed on the surface of the active material. Such a film increases the resistance between the active material and the electrolyte and is, therefore, considered as a major cause of a reduction in the charge/discharge cycle life of the battery.
To solve the above-described problem, a method of preventing a negative electrode active material from cracking by using SiOx (0<x<2) having an expansion coefficient during charge lower than that of silicon has been proposed (see Japanese Patent Laid-Open No. 6-325765).
A method of producing a silicon oxide having a low expansion coefficient has also been proposed (see Japanese Patent Laid-Open No. 2002-260651). According to Japanese Patent Laid-Open No. 2002-260651, silicon and silicon dioxide for example are mixed with each other and heated to generate SiO gas; the generated SiO gas and oxygen gas are mixed with each other; and the oxygen ratio x in SiOx is controlled to 1.05 to 1.5.
However, Japanese Patent Laid-Open No. 6-325765 includes no concrete description of an embodiment in which the oxygen ratio x in SiOx is controlled so as to satisfy 0<x<1. The inventors of the present invention have further tested several methods described in the above publication as examples of production methods to find that none of them ensures that the oxygen ratio x in SiOx cannot be uniformly controlled so as to satisfy 0<x<1.
For example, Japanese Patent Laid-Open No. 6-325765 discloses a method in which silicon dioxide and silicon are mixed with each other at a predetermined molar ratio and the mixture is heated in a nonoxidizing atmosphere or a vacuum. For example, if SiO2 and Si are mixed and heated under reduced pressure, SiO gas is generated. SiO is produced by cooling SiO gas. When SiO is exposed to the atmosphere, the surface of SiO is oxidized by oxygen gas in the atmosphere and the molar ratio x of oxygen becomes higher than 1. That is, SiO is obtained and the obtained SiO is oxidized to increase the molar ratio x of oxygen. But the molar ratio x of oxygen cannot be reduced to 1 or less.
Japanese Patent Laid-Open No. 6-325765 also discloses a method in which SiO2 is reduced by being mixed with carbon or a predetermined metal to control the oxygen ratio x. However, it is difficult to reduce SiO2 so as to obtain the desired uniformity in oxygen ratio x. Therefore, SiOx cannot be obtained with a constant distribution of the oxygen ratio x. If the oxygen ratio x varies among different electrode plate portions, the amount of absorption of Li and the expansion coefficient when Li is absorbed vary, resulting in nonuniformity of the charge/discharge reaction in the electrode plate and deformation of the electrode plate.
In such a case, carbon or a metal used as a reducing agent remains as an impurity in its original form or in the form of a chemical compound such as SiC or SiMx (M: metal) in the electrode plate. Such an impurity has lower reactivity with lithium in comparison with SiOx and therefore reduces the capacity of the negative electrode.
Japanese Patent Laid-Open No. 6-325765 further discloses a method of oxidizing silicon by heating silicon together with oxygen gas. By this method, however, SiOx is generated inwardly from the silicon surface. Therefore, SiOx and an unoxidized Si portion coexist in each particle and it is not possible to form SiOx particles having a uniform oxygen distribution.
Each of lower silicon oxides obtained by production methods such as those disclosed in Japanese Patent Laid-Open No. 6-325765 and described above includes Si and silicon oxides such as SiO and SiO2 other than the intended lower silicon oxide, the contents of these Si and silicon oxides being higher than 1 wt %. Thus, none of the production methods disclosed in Japanese Patent Laid-Open No. 6-325765 makes it possible to produce a high-purity silicon oxide.
The method disclosed in Japanese Patent Laid-Open No. 2002-260651 enables production of SiOx controlled so that the oxygen ratio x is 1.05 to 1.5, but does not enable the oxygen ratio x to be reduced to 1 or less. Also, the amount of absorption of Li in SiOx is small when the oxygen ratio x is 1.05 to 1.5. For this reason, the capacity of a negative electrode using the above-described silicon oxide as an active material is smaller than that in the case of using SiO.
Further, a negative electrode containing SiOx in which the oxygen ratio x is 1.05 to 1.5 has a large irreversible capacity and consumes part of the capacity of a positive electrode. The battery capacity is considerably reduced thereby.
For these reasons, the negative electrode described in Japanese Patent Laid-Open No. 2002-260651 is incapable of utilizing the characteristics of high-capacity silicon and obtaining the expected capacity.